Three-Coordinate Iron(II) Expanded Ring N-Heterocyclic Carbene Complexes

A sterically demanding seven-membered expanded ring N-heterocyclic carbene (NHC) ligand allows access to
rare examples of three-coordinate iron(II)−NHC complexes incorporating only halide coligands of the general formula
[Fe(NHC)X2] (NHC = 7-DiPP; X = Br (1)Cl (2)). Reducing the steric influence of the ancillary NHC ligand through
modulation of the N-aryl substituents leads to either four- or three-coordinate complexes of the general formula
[Fe(NHC)Br2(THF)] (3) or [Fe(NHC)Br2] (4) (NHC = 7-Mes), dependent upon the solvent of recrystallization. The
further reduction of NHC steric influence results in four-coordinate geometries at iron in the form of the dimeric species
[Fe(NHC)Br(μ-Br)]2 (5) or [Fe(NHC)Br2(THF)] (6) (NHC = SDiPP), again dependent upon the solvent of recrystallization.
Compounds 1−6 have been analyzed by 1H NMR spectroscopy, X-ray crystallography, elemental microanalysis, Mossbauer
spectroscopy (for 1 and 3−5), and Evans method magnetic susceptibility. In addition to these measurements the three
coordinate species 1 and 4 have been further analyzed by SQUID magnetometry and CASSCF calculations, which show
significant magnetic anisotropy that is extremely sensitive to the coordination geometry